The compartment of a pickup truck is open and thus has to be covered with a compartment cover to protect articles inside the compartment. An existing compartment cover system for covering the compartment of a pickup truck typically includes a covering layer for covering the compartment of the pickup truck with a front frame edge connected at the front end of the covering layer and a back frame edge connected at the back end of the covering layer, and also includes a driver-side frame edge and a passenger-side frame edge provided respectively at the top on both sides of the compartment of the pickup truck, where both ends of the front frame edge are fixed respectively at the head of the driver-side frame edge and the head of the passenger-side frame edge, and a fixing mechanism is provided at the tails of the driver-side frame edge and the passenger-side frame edge. In use the back frame edge is pulled backward so that the covering layer covers the compartment and then both the ends of the front frame edge is fixed on the fixing mechanism. The fixing mechanism is typically structured so that a buckle base is provided respectively at the tails of the driver-side frame edge and the passenger-side frame edge, where the buckle base include a buckle which is provided with a opening faced backward and into which a buckle tongue provided on the back frame edge is engaged. A drawback of the existing compartment cover system for covering the compartment of a pickup truck lies in that the tongue engaged into the buckle in the fore-and-aft direction is weakly fixed so that the covering layer may easily be lifted off.